Monday, March 03, 2008

Multicellular Bacteria

What are the key innovations that led to the evolution of multicellularity, and what were their precursors in the single-celled microbial life that existed before the metazoa? We can hypothesize at least two distinct kinds of features that had to have preceded true multicellularity.

The obvious feature is that cells must stick together; specific adhesion molecules must be present that link cells together, that aren't generically sticky and bind the organism to everything. So we need molecules that link cell to cell. Another feature of multicellular animals is that they secrete extracellular matrix, a feltwork of molecules outside the cells to which they can also adhere.

A feature that distinguishes true multicellular animals from colonial organisms is division of labor — cells within the organism specialize and follow different functional roles. This requires cell signaling, in which information beyond simple stickiness is communicated to cells, and signal transduction mechanisms which translate the signals into different patterns of gene activity.

PZ goes on to describe the genes in a single-cell eukaryote that diverged near the base of the animal phylum. The species is a choanoflagellate called Monosiga brevicollis.

It's important to note that this single-cell organism and its multicellular animal relatives form a distinct clade that is separated from the fungi and plants. Since there are multicellular fungi and multicellular plants, the evolution of multicellularity must have occurred many times.

PZ notes that choanoflagellates have primitive forms of adhesion molecules—one of the prerequisites for multicellularity in animals—but they lack some of the standard animal signalling pathways.

PZ Myers is a fan of evo-devo. There are many problems with this approach to biology but one of the most irksome is the emphasis on animals as models for all of evolution and development. I've referred to this as Animal Chauvinism. In his recent posting PZ is careful not to claim that the evolution of multicellularity in animals is the model for all forms of multicellular species but unsophisticated readers might easily get the wrong impression. Let's try and make the generalization that PZ might have wanted to make.

We can agree with his statement that two requirements of multicellularity are the ability of cells to stick together and the division of labor where cells differentiate to carry out specialized functions. Lest anyone imagines that these properties were invented by animals—or even by eukaryotes—let's look at some simple multicellular bacteria.

The first example is cyanobacteria. That's a filament of Anabaena sphaerica at the top of this posting. The cells adhere to each other through a common cell wall, forming long multicellular filaments. Other species of cyanobacteria form different groups of cells; for example, Glaucocystis (upper right) has four cells together in a single sheath.

Look carefully at the Anabaena filament. Do you see the fat round cell in the middle of the filament? That's a heterocyst. It's a differentiated cell that has become specialized for nitrogen fixation. All the other cells are capable of photosynthesis but the heterocyst specializes in fixing nitrogen. This species is a bacterial example of a multicellular organism with two types of cells.

The specialization of the heterocyst means that the two types of cells have to communicate. This communication takes place via small pores in the cell wall between the cells in the filament. Signaling involves transfer of small molecules such as ATP and glutamine between the various cells. What this means is that some cynaobacteria meet the two criteria that PZ Myers lays out for the evolution of multicellularity. There's no doubt about the fact that this version of a multicellular organism predates the evolution of metazoa by about 2-3 billion years.

Under certain conditions the single cells of myxobacteria come together to form fruiting bodies that consist of hundreds of cells. In the most extreme examples, some cells form the stalk, some cells form sprangia and others form spores. These are multicellular bacteria with specialized differentiated cells.

There are many other multicelluar bacteria but these two are sufficient to illustrate the point. Cell differentiation and multicellularity are not inventions of animals. There weren't even invented by eukaryotes. Differentiation and multicellularity were invented by bacteria long before the true eukaryotes ever appeared on this planet.

7 comments:

We can agree with his statement that two requirements of multicellularity are the ability of cells to stick together and the division of labor where cells differentiate to carry out specialized functions.

That is correct by definition but sounds incomplete to me. To distinguish it from a straightforward colony of cells, a multicellular organism should function as a unit, suggesting that there is some kind of higher organization (such as feedbacks loops) and cells shoulds also have reduced ability to live independent lives.

I'm not familiar with the prokaryote examples that you mentioned. Do these cells exhibit some feedback interactions or interdependence?

It's important to note that this single-cell organism and its multicellular animal relatives form a distinct clade that is separated from the fungi and plants. Since there are multicellular fungi and multicellular plants, the evolution of multicellularity must have occurred many times...

...there are many other multicelluar bacteria but these two are sufficient to illustrate the point. Cell differentiation and multicellularity are not inventions of animals. There weren't even invented by eukaryotes. Differentiation and multicellularity were invented by bacteria long before the true eukaryotes ever appeared on this planet.

Since multicellularity has evolved a number of times independently, is multicellularity in extant eukaryotes "invented" by bacteria (thus a synapomorphic feature) or could it have arisen via convergent evolution?

Doesn't this also have something to do with the unit of reproduction? Seems to me that a group of independently-reproducing cells that can co-operate isn't the same thing as a piece of multi-cellular geometry that faithfully and reproducibly copies itself.

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Principles of Biochemistry 5th edition

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Some readers of this blog may be under the impression that my personal opinions represent the official position of Canada, the Province of Ontario, the City of Toronto, the University of Toronto, the Faculty of Medicine, or the Department of Biochemistry. All of these institutions, plus every single one of my colleagues, students, friends, and relatives, want you to know that I do not speak for them. You should also know that they don't speak for me.

Superstition

Quotations

The old argument of design in nature, as given by Paley, which formerlyseemed to me to be so conclusive, fails, now that the law of natural selection has been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows.

Charles Darwin (c1880)Although I am fully convinced of the truth of the views given in this volume, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. It is so easy to hide our ignorance under such expressions as "plan of creation," "unity of design," etc., and to think that we give an explanation when we only restate a fact. Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject the theory.

Charles Darwin (1859)Science reveals where religion conceals. Where religion purports to explain, it actually resorts to tautology. To assert that "God did it" is no more than an admission of ignorance dressed deceitfully as an explanation...

Quotations

I have championed contingency, and will continue to do so, because its large realm and legitimate claims have been so poorly attended by evolutionary scientists who cannot discern the beat of this different drummer while their brains and ears remain tuned to only the sounds of general theory.

The essence of Darwinism lies in its claim that natural selection creates the fit. Variation is ubiquitous and random in direction. It supplies raw material only. Natural selection directs the course of evolutionary change.

Rudyard Kipling asked how the leopard got its spots, the rhino its wrinkled skin. He called his answers "just-so stories." When evolutionists try to explain form and behavior, they also tell just-so stories—and the agent is natural selection. Virtuosity in invention replaces testability as the criterion for acceptance.

The first commandment for all versions of NOMA might be summarized by stating: "Thou shalt not mix the magisteria by claiming that God directly ordains important events in the history of nature by special interference knowable only through revelation and not accessible to science." In common parlance, we refer to such special interference as "miracle"—operationally defined as a unique and temporary suspension of natural law to reorder the facts of nature by divine fiat.

Quotations

My own view is that conclusions about the evolution of human behavior should be based on research at least as rigorous as that used in studying nonhuman animals. And if you read the animal behavior journals, you'll see that this requirement sets the bar pretty high, so that many assertions about evolutionary psychology sink without a trace.

Jerry Coyne
Why Evolution Is TrueI once made the remark that two things disappeared in 1990: one was communism, the other was biochemistry and that only one of them should be allowed to come back.

Sydney Brenner
TIBS Dec. 2000
It is naïve to think that if a species' environment changes the species must adapt or else become extinct.... Just as a changed environment need not set in motion selection for new adaptations, new adaptations may evolve in an unchanging environment if new mutations arise that are superior to any pre-existing variations

Douglas Futuyma
One of the most frightening things in the Western world, and in this country in particular, is the number of people who believe in things that are scientifically false. If someone tells me that the earth is less than 10,000 years old, in my opinion he should see a psychiatrist.

Francis Crick
There will be no difficulty in computers being adapted to biology. There will be luddites. But they will be buried.

Sydney Brenner
An atheist before Darwin could have said, following Hume: 'I have no explanation for complex biological design. All I know is that God isn't a good explanation, so we must wait and hope that somebody comes up with a better one.' I can't help feeling that such a position, though logically sound, would have left one feeling pretty unsatisfied, and that although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist

Richard Dawkins
Another curious aspect of the theory of evolution is that everybody thinks he understand it. I mean philosophers, social scientists, and so on. While in fact very few people understand it, actually as it stands, even as it stood when Darwin expressed it, and even less as we now may be able to understand it in biology.

Jacques Monod
The false view of evolution as a process of global optimizing has been applied literally by engineers who, taken in by a mistaken metaphor, have attempted to find globally optimal solutions to design problems by writing programs that model evolution by natural selection.